1. Field of the Invention
The present invention relates to a mobile radio network comprising a plurality of base stations in a mutual three-dimensional arrangement in the manner of a cellular system, wherein a respective, rigidly prescribed plurality of radio zones immediately adjacent to one another forms a radio zone group in which the frequencies channels available overall are repeated, and wherein: further, at least the radio organization-oriented signaling is undertaken in digital form between the stationary base stations and the mobile subscriber stations via organization channels designed for duplex operation and a measurement receiver having a control incorporated into the synchronous system of the base stations is provided in each of the base stations having a central control unit for radio data control.
2. Description of the Prior Art
A mobile radio network of the type generally set forth above in the German Pat. No. 30 12 484 C2, fully incorporated herein by this reference. A monitoring of the radio channels of neighboring radio regions thereby occurs by measuring field intensity and range and the storage of the field intensity values or of both values and the switching of mobile subscribers into another radio region occurs by way of a corresponding control.
A problem area arises in the design of the radio regions in that, on the one hand, the ground-bound radio propagation dependent on terrain and the nature of the built-up area leads to irregular field intensity distributions and the radio region boundaries but, on the other hand, the radio regions must be designed according to the distributions of the radio traffic density.
Since full coverage of the radio service must also be promoted, and this is particularly true in built-up regions, the manual overlaps of the radio regions resulting from the field intensity distribution are unavoidable in any case.
The parameters of radio propagation are given by the mean attenuation over a radio link section, which is the function of the frequency f, of a range E, of the terrain undulation and of the antenna height, by the log-normal distribution which indicates the locus-dependent fluctuations from the mean attenuation over a radio range section conditioned by the topographical structure of the terrain, as well as its vegetation and building, as well as by the Rayleigh fading which occurs as a consequence of the vehicle movement when traveling through the positional distributions arising on the ground due to multi-path propagation and leads to brief Rayleigh-distributed drops of the receivable field intensity.
In addition to the mean attenuation over a radio link section, therefore, the log-normal distribution and the Rayleigh fading must also be taken into consideration for the definition of the boundary of the service region of a radio region. For the log-normal distribution, therefore, an addition of about 15 dB for 95% local probability must be taken into consideration in accordance with the desired fringe service in the service region.
When a fringe servicing of 95% is required in a cluster region supplied with small zones and having ultra-high traffic density within a radio region defined by the traffic density, then the 50% coverage lines and the 5% coverage lines considerably transgress the service region. Mutual overlapping of field intensity service regions are unavoidable.
An optimum reuse of the available radio channels must be achieved for economical radio coverage of large cluster regions. This leads to the minimum number of necessary radio bases and to the highest channel group size within the individual radio regions.
It is meaningful, in order to achieve a favorable economic structure, to distribute the existing channel volume to a minimum number of radio zones. However, the fact is that an overshooting of the entire cluster already occurs with relatively high probability given a distribution to seven radio zones and, therefore, a high probability of common-channel interference also exists. The conditions do not change decisively, even for the less efficient distribution for more than seven radio zones. Due to this property of field intensity distribution, the measures that are inherent in a system and possible for achieving minimum common-channel interference, given the maximally-possible reuse of the radio channels, must be exhausted, particularly in small zones.
The possible, system-inherent measures for minimizing the common-channel interference, which serve exclusively for the purpose of minimizing the reuse intervals, point to exclusively employing channels only within the radio zones to be covered with respect to traffic and of suppressing their use in externally-disposed overlap regions, regardless of the signal quality attainable in such regions.
Since apparatus expense is necessary at the stationary side for the realization of the system-oriented measures, it is meaningful to employ the same only in cases of critical traffic density. The critical traffic density is reached when all available channels, within a radio zone group of seven radio zones having a maximum radius, must be assigned. Then a necessity of repeating all channels, even in the neighboring radio zone groups, exists. Noticeable, mutual overlaps which lead to common-channel interference thereby already appear as a consequence of the topographical irregularities of the terrain.
Common-channel interference is still definitely negligible insofar as the use of the channels in rural regions only occurs in traffic centers and the traffic density decreases greatly toward the fringes of the radio zones and in the overlap region. However, as soon as an approximate homogeneous traffic distribution appears in large-area industrial regions or suburban regions, the probability of common-channel interference necessarily rises given the same mean traffic density.
When the traffic density rises above the critical value, then, since all radio channels are already assigned, the only thing that can be done is to reduce the radii of the radio zones in order to achieve a higher service density. With the diminution of the radio zone radii, however, the transmission powers are also reduced in order to reduce the area coverage with respect to field intensity. In order to thereby reliably exclude coverage gaps, the transmission powers may not be reduced proportionally to the radii. The probability of common-channel interference therefore increases.
The boundaries of the radio zones can thereby fundamentally not be fixed in accordance with the respective irregularity of the coverage, but must be dimensioned in accordance with the traffic density, depending upon the available channel capacity. It is therefore necessary to be able to detect the service boundary of radio zones that is defined with respect to traffic.
Fundamentally, there are three types of radio zone boundaries that must be detected with different means, namely the coverage-qualified radio zone boundary which, due to the topographical irregularities, is in no way a coherent simplex structure, the interference-qualified radio zone boundary which is defined by the minimum signal-to-noise ratio of the received signal, and the traffic-qualified radio zone boundary.
The traffic-qualified radio zone boundary is defined by a specific traffic volume which must be serviced with adequate service quality at all locations, particularly in cluster regions where small zones are required. The coverage given service areas are thereby used in a specifically restricted area corresponding to the traffic demands and the channel utilization in the mutual overlap regions is suppressed in order to minimize the common-channel interference.